Voltages are frequently monitored relative to a predetermined voltage window range, defined by an upper reference signal and a lower reference signal. The reference voltages and the voltages to be monitored are received by comparator circuits, which produce fault indications to logic subsequent elements when one or both limits of the window range is exceeded. When the monitored voltages approach the reference voltages, the fault conditions appear erratically. To provide a more precise fault condition indication as the monitored voltages approach the reference voltage, hysteresis is frequently applied in the comparator circuit. With hysteresis, after the sensed signal exceeds the threshold to produce a fault condition, to restore a nonfault condition, the signal must be reduced by an amount greater than initially required to produce a fault.
The adjustment of the upper and lower reference voltages must frequently be simultaneous about a nominal center voltage, which is typically provided by two separate adjustments. When the circuit includes hysteresis, a change in trigger points is typically a constant voltage difference relative to variations in reference to voltage changes. Consequently, when the reference voltages are adjusted, the relative percentage or proportion of hysteresis changes relative to the reference voltages. However, if it is desirable to maintain a small, constant percentage of the reference voltages as a hysteresis change, the corresponding hysteresis circuit element must also be adjusted. Therefore, if both the upper and lower comparator circuits incorporate hysteresis, any change in reference window voltages would necessitate adjustment of both the upper and lower reference voltages and the upper and lower hysteresis adjustment. This condition presents a cumbersome problem for manufacturing and an opportunity for failure due to an excessive number of adjustable components and their misadjustment.